"I can officially say I'm not on any social network," the actress said at a festival press conference

Imitation may seem the sincerest form of career promotion for Hollywood stars.

But Keira Knightley and Benedict Cumberbatch, touting The Imitation Game on Tuesday at the Toronto Film Festival, told fans to ignore people imitating them on Twitter with fake accounts. "I can officially say I'm not on any social network. If there's a name out there pretending to be me, sorry," Knightley told the film's press conference at Bell Lightbox in Toronto.

Cumberbatch, who plays the pioneering computer scientist Alan Turing in Morten Tyldum's biopic, also said he prefers his privacy to going onto a social networking site. "If you're trying to hoodwink by pretending to be me, that's pretty cruel. I'm not on Twitter," he added.

The issue of mimicry also figured when a modest Cumberbatch talked about being cast in the roles of brilliant characters in Hollywood movies. He said it was "very flattering on one account and disturbing on the other as I'm far removed in intelligence from some of the characters I've been asked to portray."

Cumberbatch admitted to being confounded when he researched his role as Turing, a Cambridge mathematician hired by the British military to break Nazi codes. "You immediately hit walls as a dumb actor trying to understand algebra," he said.